The manufacturing of wood composite board and building materials requires the evaluation of many variables. The sizes of the wood fibers and particles, the glue content, the pressure, the pressing time, the temperature, and other factors must be determined and checked in order to produce a board of consistent quality. An accepted measure of consistency is the density profile of the board. If the density profile of a composite board with the desired strength characteristics can be reproduce,, then the strength characteristics can be maintained.
Several different methods for evaluating density profiles have been used in industry and in the laboratory. The gravimetric method is the direct method; the density measurements, in this case, can be evaluated approximately every 0.076 cm (0.030 in.) of thickness.
Two methods involving radiation sources have also have been evaluated. Parker et al. (1980) used X-rays to produce a photographic image of the profile of a composite wood specimen. This method allows for virtually continuous density measurements across the cross-section of the board, with correlations made with known standards. Laufenberg (1985) and Winistorfer et al. (1986) used a gamma ray source to scan the thickness of a specimen. The transmitted radiation was then correlated to the material density, and a continuous measurement was again possible.
Of the methods discussed, the conventional gravimetric method for density determination is the most time-consuming and is the least resolute. Although the density calculations are the most direct (weight/volume), as the specimen is planed the measurements become more difficult to obtain, due to the decreasing thickness. A thicker specimen would be easier to handle, but would also require more time, due to the greater number of sections to be removed.
The x-ray and gamma ray techniques for density measurements each provide continuous profiles of the specimen, but they also require radiation sources and a greater knowledge of equipment operation by the user. The radiation methods are faster than the gravimetric method and provide information about densities at specific points rather than averages over small sections. Both methods are claimed to be nondestructive; however, this would not be the case in practical applications in industry. The samples being evaluated must be no more than 7.62 cm (3 in.) in width; so some part of the manufactured board must be cut. The operation of these densitometers would not only require training, but would also incur a substantial initial cost before consistent measurements could be made.
We have now found that acoustic emission (AE) can be used to evaluate the density profile of a wood composite.